In recent years, hydrogen which, when burnt, is transformed into H.sub.2 O has been attracting attention as a clean energy. When utilization of hydrogen for energy is contemplated, such factors as the method for storage of hydrogen and the method for transportation thereof claim an important consideration. Since ordinary cylinders require large volumes for containing hydrogen economically, it is necessary to adopt some other methods. A hydrogen-storage alloy that reacts with hydrogen and absorbs hydrogen in the form of a metal hydroxide and, when exposed to heat, releases the hydrogen has been arresting keen attention as potential means for storage and transportation of hydrogen. The hydrogen-storage alloy promises extensive utility. Besides the means for storage of hydrogen and the means for transportation thereof mentioned above, the feasibility of utilizing the hydrogen-storage alloy in a wide variety of applications such as in fuel sources for automobiles driven by hydrogen, materials for anodes in secondary batteries, devices for separation, recovery, and refinement of hydrogen, heat pumps, quenching and warming systems, refrigerating systems, heat storing devices, actuators, compressors, temperature sensors, and catalysts is being studied.
The advance of electronic devices in recent years has come to demand secondary batteries as power sources for these electronic devices to offer improved performance. Specifically, the batteries have been required to acquire an increased capacity and an elongated service life. As respects the former requirement, in the Ni--Cd secondary batteries which have been in popular use to date, for example, nonsintered Ni--Cd secondary batteries provided with nonsintered electrodes of a three-dimensional construction have been developed to take the place of the electrode substrates in the conventional sintered batteries. These nonsintered Ni--Cd secondary batteries, however, have not necessarily attained a conspicuous increase in capacity.
In recent years, such an alkali secondary battery as, for example, a Ni-hydride secondary battery which uses as an anode what has a hydrogen-storage alloy powder immobilized in a collector has been proposed and has come to attract attention. The anode using this hydrogen-storage alloy is capable of increasing the energy density per unit weight or unit volume as compared with cadmium (Cd), a material for the anode in the typical conventional alkali secondary battery. The Ni-hydride secondary battery, therefore, is characterized by permitting an increase in the capacity of a secondary battery and having only a sparing possibility of environmental pollution as well.
As prospective materials for the construction of anodes in such secondary batteries as mentioned above, such crystalline hydrogen-storage alloys as alloys based on LaNi.sub.5 having a CaCu.sub.5 type crystal structure, alloys based on ZrNi.sub.2 having a Laves structure, alloys of the TiFe type, and alloys of the Mg.sub.2 Ni type have been mainly studied. A typical method for manufacturing an anode for an Ni-hydride secondary battery from such a hydrogen-storage alloy will be described above. First, the hydrogen-storage alloy is fused by means of high-frequency melting or arc melting and the melt is quenched and pulverized. The hydrogen-storage alloy powder thus obtained is kneaded with a conducting material and a binding agent and the resultant mix is applied or deposited fast on a collector. The anode using the hydrogen-storage alloy is consequently obtained.
The anode containing the hydrogen-storage alloy, when incorporated in a secondary battery, is fated to be immersed in a highly concentrated aqueous alkali solution serving as an electrolytic solution and also exposed to the oxygen emanating from an anode during the course of overcharge. The hydrogen-storage alloy of a crystalline texture, therefore, possibly has a serious problem of being deteriorated by corrosion. Further, since the hydrogen-storage alloy by nature undergoes voluminal expansion and contraction due to the entry therein and the release therefrom of hydrogen during the courses of charging and discharging, it has the problem of sustaining a crack and incurring progressive comminution of the hydrogen-storage alloy powder. Since the hydrogen-storage alloy consequently incurs an accelerated increase in the specific surface area thereof, the ratio of surface area deteriorated by the alkali electrolytic solution to the whole surface area of the hydrogen-storage alloy is inevitably increased to a great extent. Further, since the electroconductivity between the hydrogen-storage alloy powder and the collector is likewise deteriorated, the cycle life of the hydrogen-storage alloy is inevitably curtailed. Since the hydrogen-storage alloy of a crystalline texture which is manufactured through the aforementioned melting and pulverizing steps suffers occurrence of segregation in the texture of alloy during the course of such steps, it has the problem that the segregation is liable to induce dispersion in the electrode characteristics (particularly service life).
Therefore, it has been proposed to use amorphous hydrogen-storage alloys such as, for example, the amorphous hydrogen-storage alloys of the TiNi type and the MgNi type which are obtained by subjecting melts of corresponding alloys to rapidly quenching (U.S. Pat. No. 4,623,597 and JP-B-04-80,512). The feasibility of using amorphous hydrogen-storage alloys for the purpose of eliminating segregation, precluding the occurrence of dispersion of electrode characteristics and, at the same time, improving the resistance to corrosion, and preventing the comminution of alloy powder is being studied.
The conventional amorphous hydrogen-storage alloys mentioned above, however, have the grave disadvantage of being deficient in the plateau characteristic among the PCT (pressure composition thermostat) characteristics which have large effects on the characteristics of the alkali secondary battery. To be specific, they fail to obtain a plateau area of a small gradient and do not absorb and release hydrogen in a sufficiently large volume in the range of practical hydrogen pressure. As a result, they are no longer capable of attaining an increase in capacity as in an alkali secondary batter, for example. They further have such problems as necessitating a large number of cycles for the purpose of initial activation.
In addition to fulfilling the characteristics mentioned above, the hydrogen-storage alloy is generally required to absorb and release hydrogen at a high speed, incur only small hysteresis of the absorption and release of hydrogen, and enjoy light weight and low cost.
It is an object of this invention to provide a hydrogen-storage alloy which allows an increase in the volume of hydrogen to be absorbed and released in a practical range of hydrogen pressure and further facilitates the initial activation of the alloy besides realizing the improvement in the resistance to corrosion and the prevention of comminution of the alloy powder. Another object of this invention is to provide an alkali secondary battery which realizes both elongation of service life and enlargement of capacity in addition to securing practical serviceability.